1. Technical Field
The disclosure relates to a manufacturing method of a light-emitting diode (LED), and in particular to a method of imprinting on a LED through a soft mold, so as to improve luminance thereof.
2. Related Art
Regarding developments of commercial LEDs, the materials adopted since 1960's, such as gallium arsenide and GaInAsP, all have lower luminance and limited emitting ranges in red, yellow, orange and green colors. In 1990's, Toshiba and HP cooperated to develop high-luminance LEDs of quaternary compounds (AlGaInP), which emit high-luminance red light and amber light and commence a new wave of high-luminance LEDs. Subsequently, Nichia developed LEDs centering on indiumnitride in conjunction with organometallic vapor phase epitaxy, which can provide green light and blue light, so that the colors provided by the LED became available at all over the full spectrum, and multiple applications became possible. Meanwhile, in conjunction with proper phosphor powder, white light can finally generated by LEDs. With the improvement of the light emitting efficiency, the application of the LED has gradually expanded to conventional light sources.
The light emitting efficiency of the current nitride LED is about 20%-30%, and requires considerable effort in comparison with the 70% light emitting efficiency of the high-luminance quaternary LED. In order to extract the light from an inner area of the LED structure, a method for damaging total reflection is generally used to improve light extraction efficiency. For example, a process of wet etching or dry etching is used to roughen a surface of the LED, so as to damage total internal reflection generated on an original smooth interface, and further improve the light extraction efficiency.
However, this method typically results in undesirable uniformity of the surface structure, causing non-uniform light emitting efficiency of the entire wafer. Meanwhile, since the etching on the semi-conductor may cause a poor electrical characteristic of the LED, and a refractive index between the semi-conductor and the air varies greatly, the light extraction efficiency is limited.
In recent years, a method is proposed to imprint a micro/nano structure on the surface of the LED and etch a periodic structure on the semiconductor or a transparent conducting film, in conjunction with an etching method, so as to improve the light emitting efficiency. Since those imprinting methods mostly adopt a hard mold (for example, materials such as silicon or aluminum oxide), and an imprinting process can only be performed on a single chip, an imprinting process for a wafer level cannot be completed due to warping of the LED substrate. To meet the requirements of commercial mass production, it is necessary to perform large-area transfer to achieve an economical result. As for a single 2-inch wafer, a standard die size can be diced into over 15,000 dies, which indicates that an imprinting process of 15,000 times is required. Under such conditions, an economical result cannot be achieved.
On the other hand, due to insufficient depth of focus in current optical lithography, pattern transfer cannot be effectively performed on a warp wafer. Consequently, the conventional lithography process cannot implement pattern manufacturing on the warp or a nonplanar surface.
Therefore, the future solution of how the imprinting process can be applied to the wafer level, is the key to commercialize the imprinting process.